This invention relates to document feeding devices for image reading apparatus which read an image on each sheet of an original document while feeding it through the apparatus.
A facsimile machine is an example of the image reading apparatus that sequentially reads images on an original document while feeding each sheet of the document at a generally constant speed. The facsimile machine has a pair of upper and lower transfer rollers and a pair of upper and lower discharge rollers provided along a document path in this order. The lower transfer roller and the lower discharge roller are individually connected to a driving motor via a gear-operated transmission mechanism which is composed essentially of a combination of multiple gears. When the driving motor is run, a sheet of the document is conveyed from the transfer rollers toward the discharge rollers. The upper and lower transfer rollers, the upper and lower discharge rollers, the gear-operated transmission mechanism and the driving motor are principal components constituting a document feeding device for feeding each sheet of the document in this facsimile machine. The facsimile machine further incorporates an image reading unit located between the transfer rollers and the discharge rollers for scanning an image on each sheet of the document as it is transported by the document feeding device.
In the facsimile machine thus constructed, the document feeding speed increases momentarily, just before a rear end of a sheet of the document leaves the upper and lower transfer rollers, and then decreases momentarily immediately after the rear end of the sheet has left the upper and lower transfer rollers. The document feeding speed is not kept precisely constant in the conventional facsimile machine as described above. Fluctuation of the document feeding speed causes a deterioration of image reading accuracy, and this is a common problem encountered with the facsimile machine as well as with other image reading apparatus of the prior art.